Of all the taxonomic ranks species is the most clear, distinct, and concrete. More practically, it is the level which most naturally falls out of the patterns of life’s tree. Or does it? If the term “species concept” does not ring a bell, please see this entry. If it does, how do you define species in a non-arbitrary manner?

Partly just from reading natural history on Wikipedia, I’ve become a complete nominalist on species. There seem to be frequent adjustments at every level of the tree, e.g., I just recently found out that the order Insectivora has been abolished.

I basically support the Endangered Species Act, but some of the arguments about species vs. subspecies strike me as undecidable and arbitrary. That doesn’t mean that we can’t have a legal definition of species, derived from the scientific definition but not identical to it. But it does confuse the issue.

ackbark

What was the creature, I think in California, that was thought to be two different species based on their foraging behavior but when the dna was analyzed they were identical? (think I read about it here).

My question about it is,

do the two ever have occasion to interact, and if they do how do they get along? Do they mate as readily as they would with their foraging confreres?

Doug1

This would most definitely be a topic like most all you blog about at all frequently where I would expect you to enlighten and instruct me, rather than the other way round, but I’ll give you my somewhat informed and I hope intelligent layman’s non expert understanding.

Among sexually reproducing life forms in the animal kingdom, as I understand it there’s wide agreement among biologists and geneticists that members of a group which exclusively or by far most often reproduce with other members of that group belong to the same species. Similarly I believe all agree that members of a group which genetically can’t reproduce with members of another group, are definitely not of the same species.

The gray area comes when members of two different putative species genetically can but in reality don’t reproduce with each other or do so only vary rarely, or fairly rarely, or was rarely when geographically separated but when brought together not so rarely, as we progress from probably not to maybe are the same species, for reasons of being physically separate, not sexually attracted to each other for whatever reasons, operating in different ecological niches, and probably some other things.

I’d think that Neanderthals and Homo Sapiens Sapiens are probably best characterized as being of two different species, but also a bit gray area about that since it wasn’t impossible for them to interbreed, since there was gene introgression from Neanderthals into HSS (and maybe the other way round), we’ve recently learned. They probably were only very rarely attracted to each other, with a lot of mate guarding by HSS. I’d guess it was almost always Neanderthal male to HSS female, but wonder if you know of any genetic evidence about that.

DK

how do you define species in a non-arbitrary manner?

You don’t. Like with any other abstract concept, at some point one has to make an arbitrary decision and just agree on something by convention.

http://www.astraean.com/borderwars/ Christopher@BorderWars

The Red Wolf, an endangered species or mundane wolf x coyote cross.

John Emerson

how do you define species in a non-arbitrary manner?

You don’t. Like with any other abstract concept, at some point one has to make an arbitrary decision and just agree on something by convention.

Not every concept. When there’s a real discontinuity the dividing line isn’t arbitrary, e.g. the periodic table of elements. But with speciation you don’t have that discontinuity. Speciation is also hierarchal, so after deciding whether two individuals are one or two species you also have to decide whether a division is at the level of family, genus, or species. I know that the classification of birds and of flowering plants are both terribly messy.

DK

When there’s a real discontinuity the dividing line isn’t arbitrary, e.g. the periodic table of elements.

“Atom” or “Element” is NOT an abstract concept. They really do exists in nature. But “planets”, “color green” and “beauty” do not exist – they are our abstractions of the real world.

Onur

“Atom” or “Element” is NOT an abstract concept. They really do exists in nature.

Do they? Maybe they are just abstractions of an as-yet-unknown reality. Many things that were perceived to be very real in the past are known to be just abstract concepts today. Why not atom and element too in the future?

DK

Many things that were perceived to be very real in the past are known to be just abstract concepts today.

Examples, please. I am not getting how this can be related to an “electron”. We may change the way we describe it, but no matter what, electrons really do exist and would exist even if there were never a single conscious being thinking of an “electron”. It’s part of physical reality. Physical reality is not abstract by definition.

Onur

DK, in the past the concepts you cite as examples of the abstraction of physical reality such as planets, color green and even beauty (I can’t blame them :)) were perceived to be very real, just as gods and God were perceived to be very real in the past. My question was: Is it possible that there will be a discovery in the future that will relegate atoms, elements, molecules, protons, neutrons, electrons and other particles to the category of abstract concepts?

John Emerson

I don’t think that the idea “abstract concept” has much meaning.

As soon as mesons etc. were discovered, people started asking whether the electron was a constructed concept rather than a thing. That’s ordinary positivism .There’s nothing particularly “abstract” about species, it’s just that it’s hard to be sure where to draw the lines.

Doug1

Also since Neanderthals seem to have been quite a bit bigger than HSS, and bigger boned, I’d guess that there was a lot more death at childbirth when a Neanderthal male impregnated a HSS female – which would be another factor limiting the introgression of Neanderthal genes.

jd

As Justice Stewart said, “I know it when i see it.” The species concept is biology’s dirty little secret, as it were, because there is no generally accepted diagnosis. I tend to agree with DeQueiroz that a species can be defined as a lineage with a unique evolutionary path. How you recognize it, how you diagnose it, is the hard part that few agree on, partly because there is no good dividing line between what is a species and what isn’t. It is also partly dependent on the question one is asking. There is no species definition (or diagnosis) that is generally applicable.

Why the “dirty little secret” comment? Because understanding species is basic to a huge number of studies, is generally assumed by those not trying to define it as something biologists really understand and have a good handle on (such as the belief that it is a nonarbitrary, concrete term), and yet, for something so fundamental to the field, we can’t even define it with any level of general agreement.

jd

To answer your question more directly, I don’t think it is possible to define a species in a nonarbitrary manner, unless you restrict your definition to a particular study that is amenable to whatever definition you choose. It may be that if you limit your discussion narrowly enough, you can find a species definition that everyone will agree on and is nonarbitrary, but that definition will not be able to be generalized.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com